A compact collinear polarization gating scheme for many cycle laser pulses

We demonstrate the generation of a broadband coherent continuum extreme-ultraviolet (XUV) radiation produced by the interaction of gases with a many-cycle infrared (IR) laser field, utilizing a compact collinear many cycle-polarization gating (CMC-PG) device. The spectral width of the XUV radiation can support isolated pulses of 200 asec duration. The CMC-PG device forms a high energy content ultra-short temporal gate in a many-cycle laser pulse, within which the XUV emission is taking place. The gate width has been measured and is in agreement with the theoretical calculations. The simplicity, the compactness, the long term stability, and the high IR energy output within the gate, make the CMC-PG device an ideal tool for generating energetic isolated attosecond pulses and measure the carrier-envelope phase of a high-power many-cycle laser field.     

FEM based simulation of the pulsed laser ablation process in nanosecond fields

For the pulsed laser ablation in nanosecond fields, the key physical phenomenon of the removing process is thermal evaporation. For the process optimization of the nano-second laser ablation, it is essential to set up effective simulation that can reflect material absorption coefficient, energy intensity of laser, laser pulse shape, and so forth. In this research, material ablation in nano-second region is simulated by using a finite element method (FEM) commercial package and its result has been compared with experiment results focused on the difference in the ablation depth and its shape occurred after each laser pulse hitting. Finally, the effect of the parameter variation on the ablation process has been verified.     

INFRARED NANOSECOND LASER ABLATION OF SILICON: THE SPATIAL MULTI-PULSE ENHANCEMENT EFFECT AND ITS DEPENDENCE ON LASER PULSE DURATION – TECHNICAL COMMUNICATION

An experimental study has been performed for laser ablation of silicon at 1064 nm with variable pulse durations from 50 nanoseconds to 200 nanoseconds. A spatial multi-pulse enhancement effect has been revealed, which has been rarely reported in literature. The specific feature of this effect is that for multi-pulse laser ablation of silicon performed sequentially at a group of locations, the ablation efficiency and quality starting from the second location can be enhanced if the distance between adjacent locations is sufficiently small, and the ablation efficiency enhancement becomes more obvious as the distance decreases or the laser pulse duration increases. Further study is needed to understand the underlying physical mechanism. This effect, if well understood, can be utilized to significantly improve the quality and efficiency of infrared nanosecond laser silicon ablation. This will widen the practical applications of the low-cost and low-energy-consumption infrared nanosecond lasers and hence significantly decrease the manufacturing cost and energy consumption in many relevant areas.     

激光显示中声光调制器的研究

激光显示系统中常采用高速宽带声光调制器调制激光光强,使衍射光束的光强随视频信号变化,系统要求调制器具有很快的响应速度和很高的衍射效率。采用熔石英作为声光介质,铌酸锂作为换能器,设计并制作出工作中心频率为180MHz,调制频率为32MHz,衍射效率达75%的高速宽带声光调制器。并设计了一套使激光光源和声光调制器相匹配的光学系统,在优化激光束聚焦的基础上提高调制速度。设计的高速声光调制器基本满足了激光显示的要求。     

Acousto-optic modulator system for femtosecond laser pulses

Femtosecond laser pulses have made a revolution in multiphoton excitation microscopy, micromachining, and optical storage for their unprecedented high peak power. However, modulation of their intensity with acousto-optic modulator (AOM) is frustrated by dispersion which results in a significant stretch in pulse width. Here we report a scheme composed of two acousto-optic deflectors (AODs) to modulate the intensity of the femtosecond laser pulses with simultaneous compensation for the temporal dispersion. With commercial AODs, we demonstrated such an AOM system for the femtosecond laser pulses with overall transmission efficiency of around 80%. The pulse width of the exit beam is 115-177 fs for an input pulse of 110 fs, across the wavelength range of 720-920 nm when the temporal dispersion compensation is optimally tuned at 800 nm. The fluorescence intensity in a two-photon microscopy experiment performed using this system increased 5.5-fold over that of the uncompensated AOM.     

Creation of arbitrary spectra with an acousto-optic modulator and an injection-locked diode laser

We use a double-passed acousto-optic modulator (AOM), driven by an arbitrary waveform generator to produce multiple frequency components for a laser with arbitrary frequency spacings. A programmed sequence containing various sections of radio-frequency sinusoidal signal at different frequency is applied to drive the AOM. The diffracted light is used to injection-lock a diode laser. The combined techniques allow us to generate the multi-line spectra for the diode laser with arbitrary frequency spacings in the range of 100 MHz at a relatively high output power of 80 mW and a small power variation of 2%. Such a light source can be used in the application for laser cooling of molecules.     

3.53μm激光外差太阳光谱测量系统

激光外差技术具有高光谱分辨率特性,常用于地球大气探测研究,尤其是测量整层大气透过率及气体浓度反演。本论文设计了以窄线宽3.53μm分布反馈式带间级联激光器作为本振光源的激光外差系统,实现了整层大气中水汽和甲烷气体吸收光谱的测量,系统光谱分辨率达到0.002cm-1,信噪比为24.9dB,达到多普勒线型吸收谱线的测量要求。利用自行搭建的测量系统测量了3.53μm波段整层大气透过率,与辐射传输软件仿真分析结果进行对比,其绝对差值小于0.1,实测透过率与仿真透过率具有相同的变化趋势。该系统结合最小二乘法实现了实际大气中水汽和甲烷的同步反演,合肥地区春季水汽和甲烷的柱浓度均值分别为1.20g/cm2和1.31mg/cm2。通过对3.53μm激光外差太阳光谱测量系统的研究,掌握了调提高光谱分辨率和信噪比的方法,为获取大气分子更加准确的吸收谱线和气体浓度反演奠定了基础。     

<Emphasis Type="Bold">Cumulative Heat Effect in Excimer Laser Ablation of Polymer PC and ABS</Emphasis>

Analysis of the thermal effect and machining properties of PC (polycarbonate) and ABS (polyacrylonitrilebutadienestyrene) polymers when ablated using a KrF excimer laser is described. PC has less thermal effect on the geometric distortion than ABS in laser ablation. The cumulative heat during laser ablation results in geometric deformation in ABS micromachining. The number of laser pulses generates a greater geometric deformation in ABS than in PC according to experimental laser ablation observations. The PC ablation rate is proportional to the laser fluence, whereas ABS shows an exponential profile. The pulse repetition rate has no significant influence on PC during laser ablation, but affects the ablated patterns in ABS. During laser ablation, PC does not produce debris on the machining patterns, but debris is produced on the machining patterns with ABS.     

碳纤维增强复合材料水导激光加工实验研究

为了研究水导激光加工关键工艺参数对碳纤维增强复合材料(CFRP)沟槽截面形貌与热影响区的影响,利用水导激光加工设备设计单因素实验探究了激光功率、水射流压力、进给速度及激光重复频率四个关键工艺参数对沟槽截面形貌以及热影响区的影响规律,分析了沟槽截面形貌和热影响区形成机理。实验结果表明：激光功率对沟槽烧蚀深度和热影响区的影响最大,水射流压力对沟槽烧蚀宽度的影响最大,进给速度和激光重复频率对沟槽烧蚀深度和宽度的影响不大,对热影响区有较大影响。此外,发现沟槽截面去除区呈V字形,热影响区呈锯齿形状。通过单因素方法分析得到了较好的沟槽截面形貌,其沟槽截面烧蚀深度为772.8μm,烧蚀宽度为897.7μm,铺设方向为90°的碳纤维层热影响区为326.5μm,铺设方向为0°的碳纤维层热影响区为102.4μm。     

0Cr18Ni9不锈钢箔的飞秒激光烧蚀

利用飞秒激光对厚度为20 μm的0Cr18Ni9不锈钢箔进行了表面烧蚀、微细切割等试验,并研究了不锈钢箔的烧蚀特性。首先,根据烧蚀区域的直径和脉冲能量的关系,得到了0Cr18Ni9不锈钢箔的单脉冲烧蚀阈值,并估算了飞秒激光的束腰半径。然后,对飞秒激光切割不锈钢箔的边缘进行金相观察并测试了切割试件的电阻率,以确定飞秒激光切割对不锈钢箔的热影响。最后,对切割试件进行X射线衍射分析(XRD),以确定飞秒激光切割对不锈钢箔物相组成的影响。实验结果表明:飞秒激光的束腰半径为10.416 μm;厚度为20 μm的0Cr18Ni9不锈钢箔的单脉冲烧蚀阈值为0.455 J/cm²;飞秒激光切割后试件的金相组织处于回复阶段,因此金相组织不会产生明显变化;飞秒激光切割后不锈钢箔的物相组成没有变化,但物相的相对含量发生了改变。     

圆形光斑面积变化反函数的计算方法及其应用

基于探测圆形光斑面积变化来确定被测物体位置的光电传感器存在非线性及测量范围小的问题。文中给出了最佳逼近和近似函数拟合两类３种计算方法,来计算光斑面积的反函数,并对计算方法进行了比较。长期应用的实践证明,采用适当的计算方法能改善传感器的性能,并扩大其应用范围。     

Scanning microphotolysis: A new photobleaching technique based on fast intensity modulation of a scanned laser beam and confocal imaging

The fluorescence photobleaching method has been widely used to study molecular transport in single living cells and other microsystems while confocal microscopy has opened new avenues to high-resolution, three-dimensional imaging. A new technique, scanning microphotolysis (Scamp), combines the potential of photobleaching, beam scanning and confocal imaging. A confocal scanning laser microscope was equipped with a sufficiently powerful laser and a novel device, the ‘Scamper’. This consisted essentially of a filter changer, an acousto-optical modulator (AOM) and a computer. The computer was programmed to activate the AOM during scanning according to a freely defined image mask. As a result almost any desired pattern could be bleached (‘written’) into fluorescent samples at high definition and then imaged (‘read’) at non-bleaching conditions, employing full confocal resolution. Furthermore, molecular transport could be followed by imaging the dissipation of bleach patterns. Experiments with living cells concerning dynamic processes in cytoskeletal filaments and the lateral mobility of membrane lipids suggest a wide range of potential biological applications. Thus, Scamp offers new possibilities for the optical manipulation and analysis of both technical and biological microsystems.     

Study of nanoobjects of different nature using submillimeter laser ablation

This paper reviews the experimental work conducted at the physicochemical workstation of the Novosibirsk free electron laser (FEL) since 2005 on developing methods for studying the fractional composition of nanoobjects of different nature. It has been found that the submillimeter radiation of the FEL leads to soft ablation of biological macromolecules. Furthermore, the molecules are transferred to the aerosol phase separately, each type of molecules forms its own fraction of aerosol nanoparticles without destruction and denaturation, and their sizes correlate with molecular weight. It is suggested that submillimeter laser ablation can be used to determine the sizes of nanoobjects of different origin. The paper discusses the results of experiments with mineral clusters, nanopowders, synthetic and biological polymers carried out using traditional methods of investigation of the fractional composition of nanoscale materials (atomic force microscopy, scanning electron microscopy, dynamic light scattering, and x-ray diffractometry) and using the new method—the analysis of the aerosol products of submillimeter laser ablation. It is shown that submillimeter laser ablation with the subsequent registration of aerosol products by modern equipment is a versatile, simple, fast, and accurate method which can complement the above-mentioned methods.     

SAR光学处理器中的电子控制与显示系统

本文讨论SAR光学处理器电子控制与显示系统.这套系统中含有将张力误差控制在±10g以内的胶片张力控制,它是带有AGC的位置控制系统.确保数据输片机和图像输片机跟踪精度优于0.5%的胶片速度控制,它是由一台精密脉冲信号发生器控制的两套具有复合控制功能的锁相速度控制系统.线阵CCD成像器件与图像存贮器的有效组合,使SAR光学处理器具有快速图像显示能力.采用声光调制器(AOM)的光强稳定系统,可将激光器输出强度稳定在±2%以内.还有位置伺服式电动调焦以及附加信息的译码显示与记录等.     

Control of microstructure shape and morphology in femtosecond laser ablation of imprint rollers

With increasing demand for microstructure shape accuracy for MEMS and optoelectronic devices, controllability of shape and morphology in micro-fabrication has become increasingly crucial. In this paper, the effects of processing parameters on the shape and morphology of microstructures in femtosecond laser fabrication of imprint roller are explored. An optimized fabrication process is proposed to acquire high accuracy microstructures, in which a two-step inclination ablation process and optimal laser focus position are adopted. Adjusting and matching the processing parameters is a basic method to acquire well-defined shapes, but the ablation results indicate that the draft angle of microstructures can only be adjusted in a limited range due to the intensity distribution of laser beam. A two-step inclination ablation process is adopted to increase the draft angle. In the two-step inclination ablation process, the laser beam irradiates the target surface with an angle and the microstructure with a much steeper draft angle forms after the two-step fabrication. Laser focus position is explored as an important parameter affecting the morphology, and an optimal laser focus position is obtained to enhance the ablation quality. By matching the laser fluence and laser focus position, this morphology enhancement method can realize the high-quality ablation of microstructures with a wide range of dimensions without changing the focusing objective lens.     

Influence of laser irradiation condition on a femtosecond laser-assisted tomographic atom probe

Influence of femtosecond laser pulse condition on the performance of an energy-compensated optical tomographic atom probe has been investigated. The unstable oscillator makes the mass peaks significantly broadened. Double 80 fs pulse train with 10 ns interval makes the mass peaks slightly shifted to the higher mass side. The mass peak shift corresponds to the fight time of ions triggered by laser pulsing. Chirping ratio for the laser pulses ranging from 80 fs to 10 ps is controlled by the pulse compressor for the fragile specimens such as oxide dispersion strengthen steel or insulator materials. A first-principle calculation for optical dielectric breakdown in diamond has been successfully demonstrated. It is shown that effective conductive increase has appeared at the laser intensity around 10¹³ W/cm².     

准分子激光眼屈光手术中的一种过渡区模型

提出了一种在准分子激光眼屈光手术中过渡区模型的构造方法.首先设计一种修边函数,然后提取光学区边界的切削深度,并将其扩展到整个过渡区,最后通过修边函数和过渡区上扩展切削深度的乘积得到过渡区模型.再将该模型进行计算机仿真研究和临床研究.临床上统计了8名患者16只眼睛接受该模型指导的激光手术情况:术前平均裸眼视力为0.09±0.04,术后第1天平均裸眼视力为1.06±0.24,之后视力逐渐升高,在术后一个月时平均裸眼视力为1.42±0.36,裸眼视力最小值为1.0.仿真结果,特别是临床结果表明:过渡区模型实现了光学区与非切削区之间光滑平坦地过渡.采用参数化控制的模型,特别适用于光学区直径和过渡区宽度经常调整的情况,对各种眼屈光不正情况均适用;在光学区直径确定后,过渡区宽度的改变不影响光学区的切削深度;模型有较强的通用性,适用于传统仅矫正屈光不正(近视、远视、散光)的光学区模型,也适用于波前像差引导的激光眼屈光手术.利用过渡区模型可提高手术后患眼的视觉质量.     

High coherent bi-chromatic laser with gigahertz splitting produced by the high diffraction orders of acousto-optic modulator used for coherent population trapping experiments

To prepare the coherent population trapping (CPT) states with rubidium and cesium, the commonly used atoms in CPT studies, a coherent bi-chromatic light field with frequency difference of several GHz is a basic requirement. With a 200 MHz center frequency acousto-optic modulator (AOM), we have realized bi-chromatic laser fields with several GHz frequency splits through high diffraction orders. We have experimentally studied the coherence between two frequency components of a bi-chromatic laser beam, which is composed of ±6 orders with frequency split of 3 GHz diffracted from the same laser beam, and the measured residual phase noise is Δφ(2)<0.019 rad(2). The bi-chromatic laser fields were used to prepare CPT states with (85)Rb and (87)Rb atoms, and high contrast CPT signals were obtained. For CPT states preparation, our study result shows that it is a feasible approach to generate the bi-chromatic light field with larger frequency splits through high diffraction orders of AOM.     

Lasers in gynecology

Lasers are used in gynecology for excising lesions of the cervix, vagina, and vulva. The carbon dioxide laser is most commonly used, offering precision, minimal blood loss, and prevention of strictures. Preoperative diagnostic evaluation and visualization of the lesion is essential. Endoscopic use of the neodymium:yttrium-aluminum-garnet laser through the hysteroscope for excessive uterine bleeding, and laparoscopic use of the CO2 laser for ablation of endometriosis and associated adhesions have been successful. Photoradiation with an argon tunable dye laser has been used to destroy cancer cells containing a hematoporphyrin derivative. Because special skills are required in laser use, criteria establishing qualifications of users have been developed and are recommended for use.